Simulation theory is amazing to work with. We wouldn't die, death is a part of the simulation. There would just be nothing and then something. If our existence is just code then it doesn't matter if there is a crash as long as our code doesn't read "Death" on each of us. But you're right, if the simulation worked that way time travel would technically be possible. It's possible that every instance is saved or only specific instances are saved in the simulation. the problem is harnessing the simulation itself to abuse it. If we do that, it could do two things,
1. Give us power to alter reality
2. Destroy everything possibly permanently.
The issue also is if you go back to a save point you would no longer be living within this simulation. The reason I think it's not possible is if you think about a video game. If you're playing a game with multiple saves, the only way to access the other saves is if you're playing on that save or give that save to someone else. Meaning, past points of our universe could be playing out from other simulators with different results and realities. But, for someone in a current ongoing game to try and go back into time (to a different save) would make them cease to exist. They could succeed but unless that save is active than they are gone until it's activated.